1. Field of the Invention
This invention relates to a method and apparatus for automatically welding wires, and especially thin wires, to supports or terminals.
More particularly, this method and apparatus are designed for use in manufacturing electrical resistors of the type comprising an insulating ceramic, steatite or alumina rod, on which a spiral of resistance wire is wound and anchored at its ends to metal terminals fixed to the two ends of the rod. This anchoring is obtained by welding the ends of the spiral winding to said terminals.
Although specific reference is made hereinafter to the manufacture of electrical resistors of the aforesaid type, this reference is made by way of example only, and the same method may find application in other similar uses.
With reference to the manufacture of electrical resistors, these are constituted by an insulating rod which may be of various sizes, for example with a diameter of 3 to 20 mm and a length of 10 to 100 mm or more, and which is provided at its ends with metal terminals in the form of caps for the smaller sizes, or clips for the larger sizes.
The resistance wire wound on the rod is welded to these terminals by electric welding, said resistance wire being in the form of a strip for resistors of lower value, or in the form of a thin or very thin wire, the size of which may reach capillary dimensions, for resistors of higher value.
2. Description of the Prior Art
At the present time, these resistors are manufactured using bench coil winding machines, on which the operator electrically welds the beginning and end of the winding manually to the terminals of the insulating rod.
No special difficulty is encountered in practice with this method, with the obvious exception of the slowness of the operation and the heavy use of specialised labour, because it is always possible to repair the defective weld by moving the manual electrode slightly and remaking it, until the operator considers it satisfactory.
The object of the present invention is to enable this welding to be carried out completely automatically, in the same coil winding machine by which the resistance wire is wound.
More particularly, the method and apparatus according to the present invention are suitable for application in a turret coil winding machine of known type, which is already able to carry out automatically all the other operations involved in manufacturing said resistors, including the loading of the empty insulating rod, the winding of the resistance wire, the checking of the resistor value, and the unloading.
In tests carried out up to the present time on automatic coil winding machines, the automatic welding of resistance wires, whether in the form of strips, thin wires or capillary wires, has given rise to a series of problems.
With thin wire, for example of a few tenths of a millimeter in diameter, when it reaches the end of the winding and encounters the edge of the metal terminal (cap or clip), this edge may momentarily repel the wire and cause it to travel through a further fraction of a turn on the rod. Consequently, when the wire finally overcomes the edge and climbs on to the terminal, it is in a position which does not correspond to the theoretically required position, and generally varies quite casually. Under such conditions, it is preferred to use a welding electrode with an elongated profile, operating along an entire generating line of the terminal, i.e. an axial line portion which must of necessity be traversed by the end portion of the wire which has finished the winding, whatever--as said above--its variable axial position, so that the electrode always rests securely on the wire and the welding discharge passes through it without fail.
In the case of thinner wires, i.e. capillary wires having a diameter less than 0.1 mm, other circumstances must be considered, such as:
the possibility that the cap terminals may not have been mounted coaxially on the insulating rod,
the possible malformation of the surface of the cap terminal, e.g. barrel shaped, bell shaped or undulated,
the possible incorrect positioning of the welding electrode, in particular in a position not parallel to the axis of the cap.
Consequently the tendency, in contrast to that stated heretofore, is to prefer a welding electrode which is as reduced as possible, or nearly in the form of a point. In this respect, with an elongated electrode it would not be possible to ensure that the welding discharge takes place always exactly through the wire to be welded, rather than at points adjacent to it on the metal terminal surface, these points projecting beyond the wire on account of the small wire thickness. On the other hand, if the electrode is in the form of a point, the axial position of the wire to be welded must be sought, and this is obviously neither easy nor rapid.
With resistors constructed of larger strip wires, there is an essentially economical problem in relation to the fact that said resistors are formed from just a few turns of strip, the length of which is very reduced. In this case, the length of the strip to form the resistor is practically of the same order of magnitude as the length of strip connecting two successive resistors being formed on a winding machine, if not smaller. Thus, having welded the strip ends to the resistor terminals and cut them, the remaining length of connecting strip results in considerable scrap, which represents a substantial proportion of the total cost of the material used, and can reach more than 50% of the strip fed.